All digital computing equipment requires some type of memory storage, and most of the present day computer memories store the data in binary form on moving magnetic medium, such as a magnetically coated tape, drum, or rotating disc. Because they are unlimited in length, tape memories have a very large storage capacity potential but are considered comparatively slow because of the time required to search for particular data along the length of the tape. Disc and drum memories have very fast access to the data because, as the disc or drum rotates, the data passes under the transducer at each revolution. However, disc or drum memories have a limited capacity depending upon the number of tracks of data, the length of the tracks, and the bit density of recorded data in the tracks. In order to obtain the maximum storage capacity from a given size rotatable memory, it is necessary to select an efficient data recording system.
The simplest magnetic recording method is commonly referred to as the "return to bias" method, which records on the magnetic medium a pulse representing a binary "one" and the lack of a pulse representing a binary "zero." Although a simple and inexpensive system, this method of recording is not widely used because the two flux changes required for recording each bit produces a relatively slow recording system and also because the absence of any recording represents a binary zero and thus may result in readout error.
The possible errors introduced by the lack of a signal being read as a binary zero is overcome by a recording system referred to as the "return to zero" method of recording, in which a binary one is represented by a recorded pulse of one polarity and a binary "zero" by a pulse of the opposite polarity. While solving the problem of possible readout error from lack of signal, this method of recording is relatively slow and not widely used because it is, again, a double transition method requiring two flux changes per recorded bit.
A system which apparently obviates all of the disadvantages referred to above is the "non-return to zero" (NRZ) method, which is fast in that there is a maximum of one flux change per bit, i.e., the transducer current switches only when a binary "one" is recorded. Although very popular, this NRZ method has its disadvantages. Because there is not always an output for each bit sensed by the transducer, the method is not self-clocking and it is, therefore, necessary to record a clock track along with the data tracks. Furthermore, the method is subjected to amplitude dependent time errors, that is, since data is contained only in flux changes, the amplitude of the read-back signal will vary with the data pattern. Another problem of NRZ recording is associated with the existence of high frequency noise at the baseline of the signal in patterns that contain fewer flux changes. The existence of this type of noise increases the error probability and the necessary complexity of the read amplifier design.
Still another method of recording is known as "phase modulation recording" in which the recording current wave form consists of a series of complete cycles a "one" differing from a "zero" only in phase. Although phase modulated signals require a maximum of two flux changes per bit, it is possible to record by this method at a very high rate and at bit densities approaching that of the NRZ method of recording. Furthermore, since there is an output signal for each recorded bit, this system can be made self-clocking and the output information can be correctly interpreted without the necessity of a separately recorded clock signal, as is required in the NRZ method.
The invention disclosed in the referenced patent provided a method and circuitry for doubling the bit density of a phase modulated data signal and, therefore, the memory capacity of a magnetic recording medium. The invention in the referenced patent accomplished this in circuitry that accepts a binary input signal, converts it into a phase modulation double pulse signal, and then modifies that signal into a single pulse signal which may be recorded at high-bit densities. The original input signal is then reconstructed in the demodulation circuitry by first shaping and amplifying the playback signal read by the magnetic transducer, detecting the edges of the shaped signal, and then gating those edges with a self-clocking signal. The invention in the referenced patent generated the self-clocking signal by circuitry including a voltage controlled oscillator operating in a phase locked loop at a frequency that is eight times the master clock frequency, a four-bit binary counter operated by the voltage controlled oscillator and reset by pulses representing the edges of the read-back signal, and a decoding matrix coupled to the output of the counter for generating pulses that are substantially one-quarter, three-quarters, one and one-quarter, and one and three-quarters of the master clock bit time.